A Dual-Color Two-Photon-Active Cavitand for Guest-Responsive Sensing of Benzene and Pyridine Vapors

Integrating precise molecular recognition with nonlinear optical functionality represents an attractive yet rarely realized strategy for supramolecular sensing with the optical readout. In this work, a two-photon-active luminescent cavitand, tris(quinoxaline)-functionalized acridine cavitand (TQAC), for the guest-responsive aromatic vapor detection was reported. TQAC was constructed through a macrocycle-to-cavitand conversion by incorporating electron-deficient quinoxaline panels into an electron-rich acridine-based macrocyclic scaffold, giving rise to an intramolecular donor-σ-acceptor framework and pronounced charge-transfer luminescence. Depending on the crystallization conditions, TQAC forms two solid-state polymorphs with distinct conformations and packing modes, affording porous TQACα and interlocked TQACβ, which emit yellowish-green and orange fluorescence, respectively. Notably, both polymorphs exhibit two-photon-excited emissions, making TQAC the first solid-state cavitand system showing dual-color two-photon-absorption property. Moreover, the guest-accessible TQACα phase displays rapid and visually distinguishable fluorescence responses toward benzene and pyridine vapors, with detection limits of 7.5 and 3.3 mg L^-1, respectively. This work establishes two-photon-active cavitands as a promising platform for supramolecular vapor sensing.